I came across this and updated my GA app... works well
. You can even setup what to say within shortcuts like you can others (it does't force "OK Google"). Definitely a work-around but probably the best you will get when it comes to that lol.
https://www.theverge.com/2018/11/20/18105693/google-assistant-siri-shortcuts-feature-iphone-ios
https://www.theverge.com/2018/11/20/18105693/google-assistant-siri-shortcuts-feature-iphone-ios

